An indoor exercise bike (i.e., a stationary bike) allows a user to get exercise within limited space as if the user rides on an exercise bike on the road and performs pedaling activities. The conventional stationary bike enables the user to manually adjust or set up the resistance level (intensity) of the pedaling activity. However, a normal user or an inexperienced user is often unable to determine the proper resistance level. Once the user gets the exercise when the improper or excessively large resistance level is given, the user may not achieve the desired effects. What is more, the user may suffer from injuries resulting from the exercise. From another perspective, the conventional exercise bike may not be able to instantly and spontaneously adjust the resistance level of the pedaling activity according to the user's physiological changes and the rate of perceived exertion regarding the user's physical activity.